Technical Field
The invention relates to a method of solving a control problem in a processing plant, in accordance with the preamble of claim 1.
Related Art
Methods of solving a control problem in a processing plant, for example in an ethylene plant for generating ethylene from crude petroleum, are known from the prior art. Such industrial control problems require a plurality of manipulated variables to be modified in order to switch the processing plant from one process state to another process state, but the modification of the manipulated variables affects one or more process variables, for example pressures in pipelines of the processing plant, which provide a crucial boundary condition for switching the plant from one state to another. The modification of the two manipulated variables must accordingly be controlled in such a way that one or more process variables remain within their permissible limit ranges during the switch between the states. For that purpose, the prior art employs generally sufficiently well-known open-loop and closed-loop control concepts, for example PID controllers, to modify the manipulated variables. The manipulated variables in question can be associated with any desired processing apparatus, especially valve devices, such as butterfly valves, gate valves, stopcocks or valves of any kind. The manipulated variables can therefore be, for example, the travel of a valve or gate valve, so that the manipulated variable determines, for example, the throttling of a flow of liquid or gas, especially in a pipeline of a processing plant.
It is usual for certain manipulated variables in a processing plant to be regarded as reference variables or to be set to a constant value in order to simplify the control problem and to use simple structures, such as, for example, PID controllers, which influence the manipulated variable in question. It is often the case that attention is not paid to the effect on particular characteristics or modifiable boundary conditions of the components in question or of the process. In particular, industrial valve devices and butterfly valves, together with their drive means, are important manipulated variables in the context of the problems mentioned above but are subject to relatively high stress, depending upon the control concept. For example, in the context of a known control variant having two cracked-gas gate valves in the ethylene process, a drive means for modifying a manipulated variable is constantly in operation, while a further drive means of a valve device keeps a process pressure within permissible limits by means of control movements. The latter valve device is thereby subjected to excessive stress which can result in reduced service life and faults in the processing plant.
The problem of the present invention is therefore that of providing a method of solving a control problem in a processing plant which takes account of significant factors, such as the mechanical characteristics of the processing units, such as valve devices or changes in process characteristics, and thus increases the service lives of the processing units with which the manipulated variables are associated and reduces the likelihood of their malfunctioning.